fix[BE]: validate query page/limit defaults and add exists helpers

internal/common/repository/common.exists.repository.go

@@ -2,6 +2,7 @@ package repository
 
 import (
 	"context"
+	"fmt"
 
 	"gorm.io/gorm"
 )
@@ -32,3 +33,21 @@ func ExistsByName[T any](ctx context.Context, db *gorm.DB, name string, excludeI
 	}
 	return count > 0, nil
 }
+
+func ExistsByField[T any](ctx context.Context, db *gorm.DB, field string, value any, excludeID *uint) (bool, error) {
+	if field == "" {
+		return false, fmt.Errorf("field is required")
+	}
+	var count int64
+	q := db.WithContext(ctx).
+		Model(new(T)).
+		Where(fmt.Sprintf("%s = ?", field), value).
+		Where("deleted_at IS NULL")
+	if excludeID != nil {
+		q = q.Where("id <> ?", *excludeID)
+	}
+	if err := q.Count(&count).Error; err != nil {
+		return false, err
+	}
+	return count > 0, nil
+}